In recent years, scanning electron microscopes (SEMs) have been employed to measure the size of or to inspect for defects in semiconductor device patterns. By way of example, critical dimension SEMs (hereinafter CD-SEMs) are used to measure the gate size of semiconductor devices, and defect inspection SEMs are used for defect inspection. In addition, scanning electron microscopes are beginning to be used for inspecting the continuity of deep holes for wiring using potential contrast.
Since CD-SEMs are used to measure the size of device patterns in semiconductor fabrication lines, improvements in not only their performance as electron microscopes, e.g., resolution, critical dimension reproducibility, etc., but also in throughput are extremely important. Although there are several factors that determine throughput, of particular importance are the movement speed of the sample stage on which a wafer is placed, and the time it takes for auto-focus when obtaining an image. In order to improve these two aspects, an electrostatic chuck is used as a means for securing a wafer to the sample stage.
Electrostatic chucks are advantageous in that they are capable of securing wafers stably. Thus, wafers are prevented from falling off the sample stage, and it becomes possible to transport wafers at high accelerations, and high speeds. In addition, since electrostatic chucks attract the entire surface of a wafer with a substantially even force, it is possible to flatten wafers even if they are warped. Accordingly, the time it takes to determine the value of the current to be passed through the coil of the objective lens for focusing, that is, the auto-focus time, may be shortened.